New measurement and control technology in tunnelling - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 270

Figure 6-28 Display of the machine position and
direction in relation to the tunnel axis or the planned
correction curve at the control position.

This enables the machine driver to continuously see the reaction of the machine to the
steering actions. This guarantees the appropriate steering commands for the situation with
continuous driving along the design axis.
The distance between laser and target is normally automatically electro-optically meas-
ured and updated with a new tunnel survey after each move forward of the laser station.
A normally available laser can be used, although it has to be moved forwards from time
to time, especially when driving curves. The resulting alteration of the laser direction has
to be inputted into the system manually. For fully automatic tracking of the laser, a servo-
totalstation can be used. The laser is automatically tracked until a change of position is re-
quired. While a ring is being built, with the tunnel boring machine stationary, an automatic
check of direction is made by sighting a target back down the tunnel. If any deviation of
the position of the laser mounting is detected, an error is reported immediately.
When the location of the tunnel boring machine has been determined, after the most recent
stroke of a gripper machine or after the most recently installed ring with a shield machine,
the subsequent correct course for the machine can be calculated. If there is only a slight
deviation, the course is set as the current tunnel axis. If larger deviations of some centime-
tres are detected, a modern system will calculate the appropriate correction curve, taking
into account the practically possible steering corrections of a tunnel boring machine and
the geometry of the segment rings (taper of the rings, see Volume 1, Section 2.8.5) in order
to ensure installation of the segment rings without constraints.
The correction curve starts from the last actually driven curve and drives the tunnel boring
machine at a feathered connection back to the design alignment. Excessive, sudden cor-
rections can easily lead to leaving the design axis on the other side.

6.6 Steering of small diameter tunnels

6.6.1 General
For the driving of small diameter tunnels, particularly for pipe jacking, special conditions
apply in many regards. The temperature differences between the heat from the machine
and the cold pipe can form air layers and turbulence, which can make the use of a laser
particularly difficult. On the other hand, the precision requirements are more stringent for